The operators of Calvert Cliffs nuclear power plant in southern Maryland shut down one of its two reactors Thursday after a control rod assembly malfunctioned during testing, federal safety regulators announced.

The shutdown occurred without complications and did not pose any risks for the public or plant staff, Nuclear Regulatory Commission spokesman Neil Sheehan said.

Unit 2 was powered down around 5 p.m. Thursday after operators checking a control rod "received some indication that electrical maintenance was needed on one of the assemblies," said Kory Raftery, plant spokesman. Exelon Corp. shares ownership of the plant in Lusby with French energy firm Electricite de France.

According to a report of the incident filed with the NRC, a control rod "dropped" into the reactor core while being tested and could not be withdrawn, requiring the unit to be shut down.

The plant's 77 control rods, made of materials that can absorb neutrons, are used to control or halt the fissioning of atoms. If the mechanism holding the rod loses power, it will release and the rod will drop into the core, Sheehan explained. Since fully inserting rods in the reactor shuts it down, it’s safer to have them drop into the core unexpectedly than not to be able to withdraw one, he said.

It was the fourth unplanned outage in the past 13 months at the facility, three of them involving control rods. The plant's two reactors produce more than 1,700 megawatts of electricity, enough to power more than one million homes.

Unit 1 had to be shut down on Aug. 12, 2012, after a control rod unexpectedly dropped into that reactor's core. An investigation determined the incident stemmed from a failure in the gripping system used to hold the rod out of the core, the NRC spokesman said.

On Nov. 26, operators testing a control rod discovered a “degraded” electrical coil powering the gripping mechanism, and shut Unit 1 down to replace the defective element, according to Sheehan.

And on May 9, Unit 2 powered down after the turbine that generates electricity from the reactor shut down. Valves feeding steam to the turbine closed unexpectedly, triggering the outage.

Raftery said a preliminary investigation indicated the problem that caused Thursday’s dropped control rod in Unit 2 was unrelated to the earlier dropped rod.

"The control rod drive mechanisms are complex and do have multiple parts," Sheehan said in an email.

While acknowledging the parts involved may be different, he said further investigation is needed to determine the cause of the latest failure. Overall, he noted, the number of unplanned outages at Calvert Cliffs remains below what would trigger increased federal scrutiny.

Though infrequent, control rod drops similar to those at Calvert Cliffs have occurred at other reactors, Sheehan said. The rods’ electrical coils and wiring are subject to intense heat on top of the reactor vessel head, he said. While acknowledging that "age-related failures can occur," Sheehan said the rods are inspected regularly.

Raftery said Unit 2 would be restarted once repairs had been completed and testing conducted.

Sheehan said federal inspectors assigned to Calvert Cliffs would monitor the plant's investigation to be sure the cause of the dropped rod is fully understood and steps taken to prevent another failure. Regulators also will be on the lookout if any issues surface that might apply to other similarly designed plants, he said.